As a producer, Pappalardi is perhaps best known for his work with Britishpsychedelicblues-rockpower trioCream, beginning with their second album, Disraeli Gears. Pappalardi has been referred to in various interviews with the members of Cream as "the fourth member of the band" as he generally had a role in arranging their music. He contributed instrumentation for his imaginative studio arrangements and he and his wife, Gail, wrote the Cream hit "Strange Brew" with Eric Clapton. He also produced The Youngbloods' first album.

As a musician, Pappalardi is widely recognized as a bassist, vocalist, and founding member of the Americanhard rock band/heavy metal forerunner Mountain, a band born out of his working with future bandmate Leslie West's soul-inspired rock and roll band The Vagrants, and producing West's 1969 Mountain solo album. The band's original incarnation actively recorded and toured between 1969 and 1971. Felix produced the band's albums, and co-wrote, and arranged a number of the band's songs with his wife Gail Collins and Leslie West.

The band's signature song, "Mississippi Queen" is still heard regularly on classic rock radio stations. They also had a hit with the song "Nantucket Sleighride" written by Pappalardi and Collins.

Felix generally played Gibson basses live and on Mountain's recordings. He is most often shown with an EB-1 but there are photographs of him playing an EB-0 live. He was known for playing a Gibson EB-1 violin bass through a set of Sunnamplifiers that, he claimed, once belonged to Jimi Hendrix.[citation needed]

Pappalardi was forced to partly retire because of partial deafness, ostensibly from his high-volume shows with Mountain. He continued producing throughout the 1970s and released a solo album and recorded with Japanese hard rock outfit Kazuo Takeda's band The Creation (old name Blues Creation). In May 1973, the British music magazine, NME, reported that Pappalardi was playing bass on, and producing former Stone the Crows singer, Maggie Bell's debut solo album, Queen of the Night.[2] In reality he did neither, as the album was eventually produced by Jerry Wexler, and William Salter and Chuck Rainey played bass.[3]